1. Field of the Invention
This invention relates to structures which prevent tampering with or reverse engineering of integrated circuits.
2. Description of Related Art
Integrated circuit (IC) manufacturers and users have several reasons for wanting to protect the contents of ICs. Some manufacturers would like to stop reverse engineering and copying of circuit designs. Some users would like to prevent copying or changing of software and information stored in ICs. In either case, there is a need to provide structures on ICs that make disassembly and reverse engineering more difficult.
One method of reverse engineering is to successively remove layers from an IC and examine the structure of each layer as it is exposed. Since adjacent layers in a IC often have different chemical properties, chemical processes can be chosen that remove the top layer but leave underlying layers intact. In this way, the layers of an IC can in effect be peeled off and the IC's structure determined.
Stored information can also be determined by disassembling an IC either by determining structure or measuring stored charge. In EPROMs, E.sup.2 PROMS, and similar ICs, information and software is stored as trapped electrical charge. If layers of an IC can be removed without significantly changing trapped charges, the information or software contained in the IC can be copied or changed.
The prior art has not provided a tamperproof IC which prevents reverse engineering or tampering with stored information. Structures are needed which make reverse engineering of ICs and tampering with ICs more difficult.